Cobalt Complexes with Pyrazole Ligands as Catalyst Precursors for the Peroxidative Oxidation of Cyclohexane: X-ray Absorption Spectroscopy Studies and Biological Applications

• Published in: Chemistry, an Asian journal Vol. 9; no. 4; pp. 1132 - 1143
• Main Authors: F. S. Silva, Telma, M. D. R. S. Martins, Luísa, Guedes da Silva, M. Fátima C., Kuznetsov, Maxim L., Fernandes, Alexandra R., Silva, Ana, Pan, Chun-Jern, Lee, Jyh-Fu, Hwang, Bing-Joe, J. L. Pombeiro, Armando
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01-04-2014; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Catalysis; Cell Survival - drug effects; Chemistry; cobalt; Cobalt - chemistry; Coordination Complexes - chemical synthesis; Coordination Complexes - chemistry; Coordination Complexes - toxicity; Crystallography, X-Ray; cyclic voltammetry; cyclohexane oxidation; Cyclohexanes - chemistry; Electrochemical Techniques; HCT116 Cells; Humans; in vitro cytotoxicity; Ligands; MCF-7 Cells; Molecular Conformation; Oxidation-Reduction; pyrazole based ligands; Pyrazoles - chemistry; X-Ray Absorption Spectroscopy; cobalt; pyrazole based ligands; X-ray absorption spectroscopy; cyclic voltammetry; in vitro cytotoxicity; cyclohexane oxidation
• DOI: 10.1002/asia.201301331
• EISSN: 1861-471X
• ISSN: 1861-4728

[CoCl(μ‐Cl)(HpzPh)3]2 (1) and [CoCl2(HpzPh)4] (2) were obtained by reaction of CoCl2 with HC(pzPh)3 and HpzPh, respectively (HpzPh=3‐phenylpyrazole). The compounds were isolated as air‐stable solids and fully characterized by IR and far‐IR spectroscopy, MS(ESI+/−), elemental analysis, cyclic voltammetry (CV), controlled potential electrolysis, and single‐crystal X‐ray diffraction. Electrochemical studies showed that 1 and 2 undergo single‐electron irreversible CoII→CoIII oxidations and CoII→CoI reductions at potentials measured by CV, which also allowed, in the case of dinuclear complex 1, the detection of electronic communication between the Co centers through the chloride bridging ligands. The electrochemical behavior of models of 1 and 2 were also investigated by density functional theory (DFT) methods, which indicated that the vertical oxidation of 1 and 2 (that before structural relaxation) affects mostly the chloride and pyrazolyl ligands, whereas adiabatic oxidation (that after the geometry relaxation) and reduction are mostly metal centered. Compounds 1 and 2 and, for comparative purposes, other related scorpionate and pyrazole cobalt complexes, exhibit catalytic activity for the peroxidative oxidation of cyclohexane to cyclohexanol and cyclohexanone under mild conditions (room temperature, aqueous H2O2). In situ X‐ray absorption spectroscopy studies indicated that the species derived from complexes 1 and 2 during the oxidation of cyclohexane (i.e., Ox‐1 and Ox‐2, respectively) are analogous and contain a CoIII site. Complex 2 showed low in vitro cytotoxicity toward the HCT116 colorectal carcinoma and MCF7 breast adenocarcinoma cell lines.
Oxidizing cyclohexane: CoII–pyrazole complexes are catalyst precursors for the partial oxidation of cyclohexane with H2O2. The catalytic activity tends to increase with a decrease in the oxidation potential and the absence of basic substituents. In situ X‐ray absorption spectroscopy studies suggest the involvement of a common oxidized mononuclear CoIII species. The complexes show in vitro cytotoxicity toward some human cancer cell lines.